1. Field of the Invention
The present invention relates to a magnetic sensor which serves as a magnetic field pick-up coil having a monotonic response to magnetic field strength over a wide frequency range from low to superhigh frequencies, a side-opened TEM (Transverse ElectroMagnetic) cell suitable for use in an apparatus for generating a spatially uniform and directional high-frequency electromagnetic field, and a permeameter using such a magnetic sensor and a side-opened TEM cell.
2. Description of the Related Art
The recent rapid advances in IT (Information Technology) accelerate the development of high-speed information and communication instruments as an infrastructural basis for IT. Many developed information and communication instruments have an ability to process signals in frequency ranges up to a GHz frequency range. These high-speed information and communication instruments incorporate many kinds of high-frequency electromagnetic devices including ICs (Integrated Circuits) as indispensable elements.
Magnetic materials, in particular, among various materials related to high-frequency devices will be discussed below. There is a growing demand for soft-magnetic thin films which exhibit desired frequency characteristics in the GHz frequency range. For example, soft-magnetic thin films which show a large magnetization change in a small magnetic field, for use in RF (Radio Frequency) integrated inductors or the like, are desirable, and radio wave absorbers for electromagnetic noise reduction are in need of high-loss magnetic materials having a permeability whose imaginary part has a large value.
For developing these magnetic materials responsive to a high-frequency, it is necessary to have an apparatus capable of accurately measuring a complex permeability in a high frequency range which serves as a reference for material evaluation. Some of the inventors of the present invention devised a permeameter with a standing wave type cavity of a doublet structure whose terminal end is short-circuited, for measuring a complex permeability in a frequency range up to 2 through 3 GHz, and a magnetic sensor (i.e., a shielded loop coil type magnetic sensor having a triplate stripline structure) which is a key device in the permeameter with the standing wave type cavity. These-devices have already been patented as Japanese patent No. 3085651.
There are approved standards for wireless LAN (Local Area Network) for use in a 5 GHz frequency range to meet demands for broadband internet, and many chip sets and products according to such wireless LAN standards have been placed into the market recently. In the field of optical communication technology, an increasing tendency has arisen to shift 10 GHz optical communication modules into an inexpensive general-purpose device market, and efforts to standardize 10 GBE (Giga-Byte Ethernet) are in a final stage.
In view of an explosive increase of devices which handle signals whose fundamental frequencies exceed 5 GHz, attempts to develop superhigh-frequency magnetic materials having high-frequency characteristics up to about 10 GHz have been activated. Permeameters which are highly accurate and can be handled with ease are also required to operate in an increased range of frequencies up to about 10 GHz.
It is important to take into account EMC (ElectroMagnetic Compatibility) in the superhigh-frequency range for using the above devices. For quantitative EMC measurement, highly sensitive magnetic sensors capable of accurately measuring high-frequency magnetic fields are much in demand. In order to accurately measure the frequency response of a magnetic sensor or measure the permeability of a magnetic material, there is also required an electromagnetic field generating apparatus for applying a known spatially uniform and directional high-frequency electromagnetic field to the device or specimen in a frequency range up to about 10 GHz.
The greatest problem which has heretofore prevented magnetic sensors from being designed and used at higher frequencies is that although shielded-loop coil type magnetic sensors are recognized in principle as being excellent for operation at higher frequencies, their accurate electric model has not been established, ideally correct behaviors of their frequency response have been unknown, and it has not been clear as to which part of the magnetic sensor practically governs limitations of their frequency characteristics.
Specifically, while a physical analysis of shielded-loop coil type magnetic sensors has not been known, the magnetic sensors have been invented according to a cut-and-try application of the fabrication technology for printed circuit boards or for multilayer ceramic boards, through technical analogy of a magnetic sensor coil made from a semirigid coaxial cable.
A review of conventional shielded-loop coil type magnetic sensors which have actually succeeded in high-frequency applications indicates that rectangular sensors having a longer side of about 10 mm are operable in a high-frequency range up to about 1 GHz, and such sensors are improved on a trial-and-error basis or reduced in shape to dimensions of about several mm or less for operation in a high-frequency range up to about 3 GHz.
However, serious develop difficulties will come up if above empirical efforts are made to develop shielded-loop coil type magnetic sensors for higher performance without clarifying the physical principles of operation thereof.
Another reason why magnetic sensors or materials operable at higher frequencies cannot be realized is that there have not been available means for generating a high-frequency magnetic field whose spatial strength is constant and whose magnetic field directions are parallel to each other. In the above permeameter with the standing wave type cavity, a standing mode wave is built in the cavity, whose size limits the maximum usable frequency up to 2 through 3 GHz. A TEM wave mode propagated through a coaxial transmission line and a pseudo-TEM wave mode transmitted through the doublet cavity are quite different from each other. Therefore, a connector between the doublet cavity and the coaxial transmission cable causes a large mode conversion loss, making it difficult to convert high-frequency electric energy from an RF signal source efficiently into a high-frequency magnetic field.
TEM cells are used in the art as means for solving the above problem of the mode conversion loss and generating a uniform high-frequency electromagnetic field. However, the maximum usable frequency of conventional TEM cells is limited to about 1 GHz. In addition, the TEM cells have a cavity structure covered with its entire outer surface of a ground metal. Therefore, difficulties are encountered in introducing a specimen to be measured into and removing it from the cavity. For the above reasons, it has been difficult to apply TEM cells to permeameters.